I. Field of the Invention
The present invention relates to clock signal generation in a wireless communication device. More particularly, the present invention relates to a novel and improved method and circuit for synchronizing a low power oscillator with a reference oscillator in a wireless communication device utilizing slotted paging.
II. Description of the Related Art
In a typical wireless communication system, slotted paging is employed to increase the standby time, and therefore the battery life, of the mobile station. The mobile station may be, for example, a cellular or PCS telephone, or a wireless local loop (WLL) handset. For example, in the wireless telephony system disclosed in U.S. Pat. No. 5,392,287, entitled "APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE RECEIVER", issued Feb. 21, 1995, assigned to the assignee of the present invention and incorporated herein by reference, a slotted paging system is disclosed.
In the just-mentioned patent, a system is described for reducing receiver power consumption in a communication system having a transmitter and one or more receivers, in which periodic messages from the transmitter to the receiver are scheduled in "slots." Each receiver is assigned a slot during which it monitors the transmissions. The transmitter transmits messages to the receiver only during the assigned slots. The receiver is in an "active state" during its assigned slot. It may remain in the active state after its assigned slot if the message requires the receiver to perform additional actions. This active state is often referred to as the "awake" state.
During the "inactive state," which is the time period between successive occurrences of its assigned slot, the receiver may perform any action not requiring coordination with the transmitter. It may conserve power during this time by removing power from one or more components such as those used for monitoring the transmissions. This inactive state is often referred to as a "sleep" state. At a time during the inactive state shortly before the assigned slot, the receiver applies power to these components and performs initializations. These initializations may include reacquiring a pilot channel signal to which the receiver may synchronize itself if its timing signals have drifted out of synchronization with those of the transmitter during the preceding inactive state.
When the mobile station is in a standby mode, i.e. it is not actively engaged in a call, the standby time, and thus the battery life, is dominated by how much current is consumed while the mobile station is in the sleep mode. In the typical mobile station, about half of the average current consumption during standby mode is consumed while the mobile station is in the sleep mode.
While the mobile station is in the sleep mode, it must at least maintain a counter to meter the sleep time, so that it will "wake up" on time for its next assigned slot. In most mobile stations, a very accurate time reference, such as a Temperature-Compensated Crystal Oscillator (TCXO), is used to meter the sleep time. Additionally, other clock signal generation circuitry such as clock buffers, clock dividers, and other related clock generation elements are typically required to generate a usable and reliable clock signal from the TCXO reference. Although this results in a highly accurate clocking scheme, all of these components consume a significant amount of current; on the order of 3 mA.
In order to decrease the current consumption during sleep, and thereby increase standby time, it has been proposed that the TCXO and related clock signal generation circuitry be turned off during sleep mode, and a low power, less accurate crystal oscillator be used in its place. Because the accuracy permitted and the high frequencies generated by the TCXO matters mostly when the mobile station is awake, it has been proposed that the low power oscillator be used to meter the sleep periods, where accuracy is less critical, and high clock frequency resolution is not as important.
Additionally, low power, low frequency crystal oscillators are relatively cheap to make, and are immensely popular in such applications as wristwatches. They are, therefore, in ready supply. They also consume very little current compared to the more accurate crystal oscillators; on the order of 0.015 mA. All of these advantages of current savings and cost make them attractive for use in metering the sleep period in a mobile station.
However, these low power, low frequency crystal oscillators are also notoriously susceptible to temperature variations, varying on the order of 60 PPM over the expected operational temperature range. They are uncorrectable and uncompensated. Additionally, they are coarse in time resolution. All of these errors detract from their ability to properly monitor the sleep period in a mobile station.
One way to compensate for the errors inherent in these low power crystal oscillators is suggested by U.S. Pat. No. 5,428,820, entitled "ADAPTIVE RADIO RECEIVER CONTROLLER METHOD AND APPARATUS", issued Jun, 27, 1995, and assigned to Motorola. In this patent, the mobile station utilizes a low cost, low power, low frequency oscillator to meter the sleep period. The error inherent in the low power oscillator is accounted for by adapting the length of the current sleep period depending on the timing accuracy of the previous sleep period. In other words, if the previous sleep period was too long due to errors in the low power oscillator, the mobile station wakes up earlier in the current sleep period. In order to determine whether the sleep period was too long or short, the mobile station looks for a unique word, such as a message preamble from the transmitter, which signifies the beginning of its assigned slot. If a unique word is not received, the mobile station woke up too late, and therefore the sleep duration is decreased. If a valid unique word is received, the mobile station woke up on time or too early, and therefore the sleep duration is increased slightly.
A significant drawback of the 5,428,820 patent is that it relies on the occasional incorrect reception of the unique word to find the correct sleep period. Also if the unique word is not correctly received and demodulated, the sleep duration is changed based on the expectation that the sleep period was the wrong length. There may be several other reasons besides the duration of the sleep period that the unique word was not correctly received and demodulated, including communication channel quality conditions.
What is needed is a method and circuit for accounting for errors in the low power oscillator that avoids the limitations of the prior art, while reliably metering the sleep period in a mobile station.